When your transformer arrives on-site, various assembly and testing procedures must be carried out before placing the unit in service.

Your large MVA-sized transformer arrives on-site with its radiators and bushings in separate boxes and its tank empty. What should you know about assembling these parts? What about transferring the dielectric fluid; are there certain filling requirements and precautions that must be addressed?

These and many more questions will be answered in our discussion here.

Different configurations of unassembled transformers

When unassembled liquid-filled transformers are shipped to the installation site, they may come in the following configurations:

* Tank with transformer filled with liquid and additional fluid shipped separate.

* Tank with transformer charged with dry air or dry nitrogen and fluid shipped separate.

* Other components, such as the radiators, bushings (made of porcelain, epoxy, or a polymer), auxiliary cooling equipment (fans), arresters, and accessory devices shipped separately and noted on the bill of lading as separate items.

As the radiators are demounted for shipment, the tank, if containing a dielectric liquid, may be overfilled with insulating fluid. Tank openings are sealed with gasketed plates. After installation, radiators are to be filled with the provided fluid.

Components requiring special inspection

You should carefully check the following items for shipping damage.

* Tank should be inspected for leaks, dents, scratches, and other signs of rough handling, such as load shifting and broken bonds or broken welds of attachments that secure the transformer.

* All external accessories should be checked for breakage, loss, and leaks.

* Paint should be inspected as scratched paint can indicate other damage.

* If the transformer tank is filled with liquid, readings should be taken of the pres-sure-vacuum gauge, liquid level gauge, and top liquid temperature gauge. Also note the ambient temperature when the above measurements are taken.

When there is external damage, it may be an indication of internal damage. If damage is indicated, immediately file a claim with the carrier and contact the manufacturer. If an internal inspection is necessary, coordinate it with the manufacturer. When there is evidence of mistreatment, take precaution to prevent further damage or contamination of the insulating liquid.

Ground the lank

Once the transformer is permanently placed, before further inspection is carried out, and before assembling the unit, the tank should be permanently grounded with a correctly sized and properly installed permanent ground.

Plan for the prevention of contaminants

When you're inspecting the transformer and installing accessories, take steps to prevent tools, hardware, or other foreign matter from falling into the tank. Some transformers have failed due to such items falling on coils or in ducts and being forgotten. Develop a procedure for inventory of all tools, hardware, and any other objects used in the inspection, assembly, and testing of the unit. A checklist should be used to record all items.

If there's evidence of damage that warrants inspection of the tank's inside, it's very important that you do not remove any component from the transformer tank or open any handhole or manhole cover or other plates until the tank is at atmospheric pressure (0 psi gauge). Sudden release of pressure can dislodge an unsecured cover or plate, causing serious injury or even death. Relieve the tank pressure (or vacuum) by slowly loosening a manhole cover, shipping plate, or plug, whichever is most convenient to vent the transformer.

You should also not open the liquid-filled compartment when it's raining or if there is excessive humidity. Dry air should be pumped continuously into the gas space if the transformer is opened under conditions of humidity exceeding 70%. Liquid-filled transformers are often shipped with nitrogen in the gas space. To avoid possible suffocation, you must arrange for the nitrogen to be purged by pumping dry air into the gas space for a half hour before anyone enters the tank. Pure nitrogen will not support life and should not be inhaled. The oxygen content should be checked and is acceptable with a concentration level of 19.5% to 23.5%.

If you have to draw down the insulating liquid for inspection, make sure you have equipment for clean and dry storage of the liquid during inspection and for filtering the liquid prior to refilling the tank. It's very important all associated equipment used in the handling of the fluid (hoses, pumps, etc.) are also very clean and dry. If this equipment was used before with a different type of fluid, clean all contaminated items. When you remove the liquid, its level should not get below the top of windings.


Liquid-filled transformers may be stored outdoors upon delivery. Sufficient gas pressure must be maintained to allow a positive pressure of 1 psi to 2 psi at all times, even at low ambient temperature. The pressure-vacuum gauge, if supplied with the transformer, will show pressure variations with ambient temperature. Pressure and ambient temperature readings should be recorded regularly. Refer to manufacturer's instructions for storage of accessories. If possible, cabinet heaters should be energized to prevent condensation.

Attaching the accessories

All items removed from the transformer for shipment will be noted on the bill of lading. Inspect these items. The reassembly should be done in the order noted below.

Radiator inspection and installation.

1. Inspect the radiator(s) and flange mating surfaces for shipping damage. The radiators dissipate heat to the air and are usually relatively flat panels with a number of hollow ridges that serve as internal passageways for the fluid to travel. Occasionally, you will find some radiators comprised of vertical tubing with headers on top and on bottom, with the end of the headers serving as attachment points to the tank.

2. Make sure valves on tank flanges are closed, and remove blank shipping plates.

3. Remove blank shipping plates on radiator flanges, and inspect for moisture or contamination inside radiator headers. If the radiators are contaminated, flushing will be necessary (cleanliness and dryness are very important). If flushing is required, do not open the tank flange valves that open to the radiator(s) prior to the flushing operation. The radiator(s) should be placed at a clean, convenient location. Then, remove the top and bottom pipe plugs from the radiator headers and circulate clean insulating fluid through the radiator's main header tubes. It's best if you use a high volume pump and filter system (capacity of 40 to 200 gpm). Reverse the flushing procedures so the radiators are flushed top to bottom and then bottom to top.

4. Clean all mating surfaces on the tank and radiator flanges. Apply a small amount of rubber cement to hold gaskets in place during installation of the radiators. Inspect and reuse "O" ring gaskets on valves. Replace any that have nicks or tears.

5. Using the lifting eye(s) at the top of the radiator, attach it to the tank. Do not rest the radiator on the drain plug, as this may cause damage. If there is more than one radiator, match the numbers on each radiator with those on the tank flanges to assure proper installation. Attach radiators with mounted fans first and then the remaining radiators. Attach nuts and spring washers to the bolts and tighten them evenly, alternating across comers top and bottom until the specified torque value is obtained.

6. For transformers shipped with fluid, after removing a manhole cover, shipping plate, or plug (whichever is most convenient), vent the tank. This opening must be above oil level. Then, based on discussions with transformer manufacturers, you should open first the top and then the bottom flange valve on each radiator in succession until all valves are open. This procedure will prevent the trapping of gases or air pockets in the radiators. (Note: Some transformer manufacturers recommend the reverse sequence.) Do not allow the oil level to fall below the top of the coil clamping ring. If there is insufficient oil in the transformer, stop the radiator filling process and add oil before completing this step. After all radiators have been filled, adjust the liquid level to the proper position.

7. If the transformer is shipped dry, it will be filled with either dry air or nitrogen. With this situation, leave the radiator valves closed until the transformer is ready to be filled with fluid. Based on this condition, you should open the valves during a vacuum cycling and the filling of the liquid.

Bushing inspection and handling. Bushings are the means for allowing current to flow into and out of the transformer. They are insulators with internal conductors, and they are fragile. You should always use extra care when handling bushings.

1. Remove the cover of the bushing crates, taking care not to chip the porcelain (or epoxy or polymer) skirts on the bushings. Check the condition of all bushings. For larger, higher voltage transformers, the bushings will be made of porcelain. Lower voltage bushings, when used for indoor installations, are usually made from epoxy or polymer. If the bushing is chipped or cracked, or if there is any other apparent damage, contact the manufacturer.

2. Some oil-filled bushings should remain at a minimum of about 10 [degrees] above horizontal. This is because there is usually a nitrogen blanket above the oil that serves as a pressure buffer to compensate for changes in oil volume during temperature variations. As such, it's important to keep the nitrogen at the top of the bushing. If the angle above the horizontal is not maintained during handling, storage, or installation, refer to the bushing manufacturer's instruction manual for corrective action.

3. Remove the bushings from the crate using a sling, lifting them in the same position. Place the bushings on a suitable surface for cleaning. (The cover that was removed from the packing crate is a good base.) Use rags to cushion the bottom threads. Never rest the porcelain section of a bushing on anything. Use the bushing flange to provide support.

4. Carefully clean the bushing using a rag dampened with a fast drying solution, such as denatured alcohol. You should check with the bushing manufacturer prior to using any solvent. All bushing surfaces that will be inside the tank should be wiped clean and then dried to prevent contamination of the oil in the transformer. Do not use solvent on the sight glass, as this may damage the material.

5. Bushings with tubular stems for conductors (draw lead bushings) leave the factory with the lower stems loosely sealed. Dust, dirt, and moisture may accumulate in the conductor tube. To clean the conductor tube, draw a cloth saturated with the cleaning solvent mentioned above through the hollow conductor. Make several passes until the rag comes out clean.

6. If the bushing has a corona shield, make sure you don't dent this shield; it must be smooth and highly polished to be effective. If possible, the corona shield should also be removed and cleaned.

Draw lead bushing installation. Draw lead bushings are low ampere devices having a flexible conductor that is pulled up the tube at installation. Because these bushings are made by different manufacturers with various features, it's important to refer to the manufacturer's instructions, which are usually supplied with the transformer for additional information. A generic draw lead bushing is shown in Fig. 1.

1. Remove the bushing shipping plates, using care not to damage the gasket. The draw leads will be attached to the underside of the shipping plate.

2. Inspect the gasket and replace it if damaged. Clean the transformer bushing mounting boss and install the bushing gasket, using a small amount of petroleum jelly to prevent damage to the gasket.

3. Pass a pull wire or cord through the center tube of the bushing and attach it to the small hole in the top of the cable terminal stud. (Serving as a fish tape, this cord or wire should be at least 12 in. longer than the bushing.) Lower the bushing into the transformer opening, taking care not to scrape or damage the current transformer. Do not allow slack in the pull wire or cord, as it may allow the lead to become kinked below the end of the bushing.

4. Install the locking pin at the top of the bushing, and remove the pull wire or cord. Thread the terminal cap into position, making sure the gasket is in place. Apply a small amount of petroleum jelly to the gasket to prevent damage. Tighten the terminal cap to seal against the gasket. Torque the stud nut and cap as directed by the manufacturer's installation instruction leaflet.

5. Install and tighten the bushing flange hardware, applying even pressure to the flange. Hardware should be tightened alternating across the flange until the recommended torque value is reached. (Refer to manufacturer's torque values.)

6. Check all leads to maximize clearance to ground and to other electrical parts.

Fixed conductor bushing installation. Fixed conductor bushings have high ampere ratings. With these devices, the tube and/or rod is the conductor. As each firm's product has different features, you should refer to the bushing manufacturer's instructions for additional information.

1. Remove the bushing shipping plates, using care not to damage the gasket. Inspect the gasket and replace if damaged.

2. To install this type of bushing, if needed, lower the oil level. Do not drain oil below the top clamping ring.

3. If the transformer is shipped without oil, start flowing dry air through the unit.

4. Clean the transformer bushing mounting boss with a solvent such as denatured alcohol. Check with the bushing manufacturer before using any solvent. Contact the transformer manufacturer if there is any damage to the mounting boss.

5. Place the bushing into correct position on top of the transformer. After confirming there is breathable air inside the tank, enter the unit, or provide access to the bottom of the bushing through access covers on the sidewall of the transformer. (It may be necessary to lower the liquid level to do this.) If applicable, first install the lower bushing terminal. Then, make the lead connection to the bushing, and be sure the other end of the lead is appropriately connected to the correct winding. Check all the leads to maximize clearances to ground and electrical parts. Secure the bushing on the transformer tank by tightening all bolts uniformly in several steps. Do not attempt to pull the bolts down to the final setting on the first tightening. Allow time between each tightening to allow the gasket to seat properly. All nuts on the bushing flanges and manhole or handhole covers should be torqued to the manufacturer's recommended levels.

Installation of other accessories

Pressure-vacuum gauge. This is usually installed by the transformer manufacturer. If not, remove the 1/4-in. pipe plug located on the tank front and about 5 in. below the top cover. Install the gauge and tighten using Teflon sealing tape or equivalent.

Fans. Sometimes the fans are already attached to some of the radiators, in which case these radiators should be at the end of the series of radiators. If this is not the case, attach the fans to the radiators. Usually, instructions for doing this are provided as is the necessary hardware. Fans must be located on a radiator at the end of a series of parallel radiator panels. When the fans are connected to the control circuit, make sure you check the direction of the fan blades for correct air flow.

Rapid pressure rise relay. One of two types is provided. If a gas space pressure relay is to be used, it's normally mounted on top of the tank. If an under oil relay is used for this purpose, it is usually mounted on a side wall at about eye level. Attach the flexible lead connector to the terminals of the rapid pressure rise relay and to the control box (connections at both ends of the lead must be completed).

Lightning arresters. For installations that may expose equipment to switching or lighting impulses, lightning arresters should be used. These arresters are mounted using brackets usually provided along with instructions. Arrange to have all ground connections securely made in accordance with the NEC and applicable local codes.

Conservator tanks. Sometimes liquid-filled transformers have conservator tanks, which are auxiliary tanks on top of large units. These tanks allow the dielectric liquid to flow into and out of this reservoir as the fluid in the main tank expands and contracts. Conservator tanks are used when the main transformer tank is completely filled with liquid. Prior to assembling the conservator per manufacturer's instructions, the bladder should be checked carefully for any contamination or defects, such as a rip in the air bag. Also, proper operation of the oil level float should be verified.

Inspection and filling

You should make a final inspection of the transformer before it's energized, particularly if any work has been done inside the tank. All electrical connections should be checked for tightness. All bushings should be checked for tightness of gaskets, and all draw lead connections should be checked. Electrical clearances inside the tank should be checked. One final check should be made to ensure all tools have been removed.

After inspection, close the tank. Reinstall all hand hole (and/or man hole) covers that have been removed. All gasket groves should be cleaned, with all gaskets in the correct position. All nuts on the bushing flanges and handhold covers should be torqued to appropriate values.

Testing for leaks

The simplest method of testing for leaks is by gas pressure. The gas space in the unit should be pressurized at 5 psi with dry nitrogen with the pressure monitored for approximately 24 hrs.

A change in pressure does not necessarily indicate a leak. Any temperature increase or decrease in the transformer will result in a subsequent increase or decrease of the gas pressure in the unit. Ambient temperatures and tank pressure should be monitored for the 24-hr period.

If there is a significant drop in pressure during the 24-hr period without an accompanying decrease in ambient temperature, you must check the tank for leaks. Then, repressurize the tank at 5 psi. Using a solution of liquid soap and soft water, brush all weld joints above the oil level, bushing gasket flanges, and handhold cover gaskets. Any leaks in the gas space above the liquid will appear in the form of soap bubbles. Use chalk dust below the liquid level to check for leaks of liquid from the tank. All of the soap solution must be rinsed or wiped off with a clean wet rag before removing pressure.

If the oil level has been lowered for inspection and/or insertion of bushings, etc., or if the transformer was shipped without being completely filled with oil, the unit must be filled to the proper level before energizing. Manufacturers usually have their own procedures for proper filling and handling techniques. Carefully follow these instructions for best results.

Determining dryness

When a liquid-filled transformer is shipped to the designated site by the manufacturer, it's almost always shipped thoroughly "dry." That is, the tank, core, and coils have very little water or moisture in them, even though the assembly may be filled with a dielectric heat transfer medium, such as mineral oil, or a fluid classified as a "less-flammable, or a high fire point liquid." Every precaution is taken to ensure dryness is maintained during shipment. However, due to mishandling or other causes, moisture may enter the transformer and be absorbed by the dielectric fluid and insulation system. Because this potential problem may exist, you should determine the dielectric fluid and insulation are dry before energizing the transformer.

For transformers shipped with the core and coils immersed in a dielectric fluid, samples of the fluid should be drawn from the bottom sampling valve and tested for dielectric strength. There are different passable voltage levels depending upon the type of test done and the fluid used. (One transformer manufacturer recommends that the sample pass a test at 26kV or more for an oil-based fluid.) If the test of the dielectric strength is acceptable, if there's no evidence of free water in the oil at the bottom of the transformer, and if the insulation resistance readings are satisfactory, you can assume the insulation is dry and the transformer can be energized.

If the tests indicate low dielectric strength, an investigation should be made to determine the cause before the transformer is energized. At this point, you should take measurements of the insulation resistance level and insulation power factor and submit them to the transformer manufacturer for recommendations. A high power factor reading means there is moisture or other contamination in the fluid.

In order to obtain a uniform insulation temperature, the transformer oil should be at normal ambient temperature when the insulation resistance and the power factor measurements are made. The liquid dielectric test should use samples of the fluid taken at both the top and the bottom of the tank, and the results should accompany the power factor reading.

If the tests or visual inspection indicate the presence of moisture, the core and coils must be dried before voltage is applied to the transformer. It's recommended that various tests of the transformer, such as insulation resistance level and insulation power factor, be taken periodically during the life of the transformer for comparison purposes. You often can discover helpful trends for planning maintenance programs this way.

Final external inspection

All external surfaces of the transformer and accessories should be examined for damage that may have occurred during installation of equipment. The liquid level gauge, temperature indicators, pressure-vacuum gauge, tap changer, and other accessories should be checked for proper operation. Bushings should be checked for cleanliness and, if necessary, cleaned with Xylene or another non-residual solvent. (Xylene, a type of solvent, as well as some other types of solvents, such as alcohol, are effective cleaners. Whatever solvent is used, its toxic qualities should be considered. Check with the bushing manufacturer if you are not sure of the characteristics of a solvent.)

All liquid levels should be checked, including those in any oil-filled switches or conservator tanks, if used. The conservator tank should also be properly vented. All electrical connections to the bushings should be checked for tightness. Proper external electrical clearances should be checked. All cables or bus connected to the transformer bushings should be checked to avoid strain on the porcelain insulators. All winding neutral terminals should be checked to assure they are properly grounded or ungrounded, according to the system operation. All tank grounds should be checked. All current transformer secondaries should be checked to assure they are either loaded or short circuited. You should use caution here. The secondaries of current transformer with open circuits can achieve dangerously high potentials.

Study the transformer's nameplate data and compare it with the planned application of the unit to assure proper usage.

Surge arresters, when used, must be installed and connected to the transformer bushings/terminals with shortest possible leads. These arresters may be necessary to protect the equipment from line switching surges and lightning.

A suitable HV disconnect means must be available to deenergize the transformer in order to operate the no-load tap changer, if used. The tap changer position must match the incoming line voltage as closely as possible. The tap changer should be padlocked in the correct position for operation.

Valves should be checked for proper operation and position. Radiator valves should be open. If a conservator tank is supplied, the connection between it and the main tank should be open. The upper filter press valve should be closed. This valve is to be opened when connection is made to it via use of temporary piping or tubing that goes to an external filter press, which is a pump and filter. It maintains flow of the dielectric fluid while passing the fluid through the filter to remove impurities, including finely divided carbon and small deposits of moisture.

When filling, use filtered liquid

When filling a transformer with liquid, filtering with a filter press is recommended to prevent foreign material and moisture from entering the tank. Begin the filtering process with new filter elements and replace them frequently, depending upon the amount of moisture removed. Filter elements must be thoroughly dried and kept warm until the time used.

Liquid filling procedure

Check the dielectric strength of the liquid while it is still in containers. If free water is present, drain off the water before putting the liquid through the filter press. Continue passing the liquid through the filter press until a passable dielectric strength is reached. (One manufacturer recommends 26kV or higher for oil.) The transformer should be filled with its dielectric liquid on a reasonably clear day when the humidity does not exceed 70%. The fluid temperature must be 0 [degree] C or higher.

A high-voltage static charge can develop when oil flows in pipes, hoses, and tanks. Therefore, ground all metal surfaces, including bushings, fluid hoses (which should be conductive), and the pump during the filling procedure. As manufacturer's recommendations for filling transformers differ, it's best to follow the guidelines provided with the unit. Also, recognize that transformers equipped with a conservator require special filling techniques.

Non-vacuum filling

Where vacuum filling is not required, the tank should be filled through the upper filter press connection (the filter press valve), at the top of the tank. A second opening above the oil level should be provided to relieve the air being displaced. Full voltage should not be applied to the transformer for 48 hrs after filling.

Vacuum filling

Entrapped air is a potential source of trouble in transformers. In general, it's desirable to fill transformers with liquid under as high a vacuum as conditions permit. The transformer tank must be airtight except for the vacuum and oil connections. If it has a conservator, be careful because it will not withstand a full vacuum. After obtaining a vacuum as high as the tank construction will permit, this vacuum should be maintained by continuous pumping for at least 4 hrs to 8 hrs. The filling may then begin.

The liquid line should be connected to the upper filter press connection or other suitable connection on top of the tank, as shown in Fig. 2. The filtered liquid is admitted through the connection, with the rate of flow being regulated by a valve at the tank so the vacuum does not fall below 90% of the original value. (This value changes per manufacturer.) Any air bubbles in the liquid will explode in the vacuum, and the air will be drawn out by the vacuum pump. The vacuum should be maintained for 1 hr to 4 hrs (again, this can change per manufacturer) after the transformer is full.

The equipment needed to fill transformers under vacuum includes a vacuum pump (approximately 150 cfm, two-stage type) capable of 200 microns in the blanked off condition. (Check for proper rotation of the pump.) This will be suitable for most transformers. A booster will be helpful on large transformers. Also a filter press (approximately 30 gpm) should be used. You'll find it advisable to have extra sets of filters. Other equipment should include a vacuum gauge, vacuum pipes or flexible hoses (2 in. dia minimum), oil supply lines and a vacuum valve (for connection at top of the transformer), and either a supply of dry air or dry nitrogen, depending upon which manufacturer you've used.

The hoses should be flushed with liquid through the filter press, filled with liquid, and connected to the top filter press connection. This is to allow as little air as possible to enter the transformer.

It's recommended you arrange for some visual means to observe the fluid level so you'll know when the oil is approximately 8 in. to 12 in. from the top of the tank. This could be done by installing a sight glass at a valve near the top of the tank or by using Tygon tubing (which is transparent) from a top valve to the drain valve. Note: Should the fluid be pulled into the vacuum pump, it can cause serious damage to the pump.

Follow these precautions:

* Never apply voltage to a transformer under vacuum. A 500V insulation resistance tester may cause damage to insulation and result in a failure.

* Never leave a transformer unattended while under vacuum. A positive pressure must be applied if it must be unattended.

* Never stand or walk on the transformer tank when the unit is under vacuum.


After applying full voltage, the transformer should be kept under observation during the first few hours of operation under load. After several days, check the oil for oxygen content and dielectric strength. All temperatures and pressures should be checked in the transformer tank during the first week of operation. Except for special designs, transformers may be operated at their rated kVA if the average ambient temperature of the cooling air does not exceed 30 [degrees] C in any 24-hr period, and the altitude does not exceed 3300 ft.

Safety precautions are essential

Lethal voltages will be present inside all transformer tanks, enclosures, and at all external connection points. Make sure installation and maintenance are performed by experienced and qualified personnel. Deenergize the transformer before performing any maintenance or service work. And remember, the secondary may be alive through feedback circuits, so it's also advisable it also be disconnected.


Each transformer manufacturer produces products that have differences from units built by competing manufacturers. And they have their own instructions for installing and testing their transformers. Such instructions should be carefully followed. This is vital to assure adequate safety to personnel and equipment.

The purpose of this article is to provide general guidelines for installing and testing liquid-filled transformers for placing them in service. The article will be helpful to the reader by describing the procedures used for meeting this objective. The article is not meant to supersede manufacturer's instructions. Each manufacturer has specific warranty requirements for its transformers. For this warranty to be valid, it's important you have a complete understanding of the conditions called for and follow the requirements.